sedimentary

Chert (White-Tan variety)

Silicon dioxide (SiO2) / Microcrystalline Quartz

AI-generated identification · may be incorrect

Hardness: 7 on Mohs scale; Color: Tan, beige, or off-white; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.

Hardness: 7 on Mohs scale
Color: Tan, beige, or off-white
Luster: Waxy to dull

Identified March 21, 2026More sedimentary →

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Physical properties

Hardness: 7 on Mohs scale; Color: Tan, beige, or off-white; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.

Formation & geological history

Formed through the accumulation of silica-rich organic remains (like radiolarians or diatoms) or by chemical precipitation in marine environments where silica replaces original carbonate minerals. Common in Phanerozoic rock layers.

Uses & applications

Historically used for stone tools and arrowheads because of its sharp edges. Currently used as road aggregate, in jewelry as semi-precious gemstones (when patterned), and as a silica source in glass manufacturing.

Geological facts

Chert is so hard it can strike sparks against steel, making it vital for early 'flintlock' firearms. Due to its microcrystalline structure, it breaks into razor-sharp edges which led to its extensive use in the Stone Age.

Field identification & locations

Identify by its waxy texture and shell-like (conchoidal) fracture pattern. It cannot be scratched by a steel knife. Common in limestone or chalk beds globally, specifically throughout the Midwestern and Western United States.